Bi-directional DC-DC converter and method for controlling the same

ABSTRACT

A bi-directional DC-DC converter has a transformer for connecting a voltage type full bridge circuit connected to a first power source and a current type switching circuit connected to a second power source. A voltage clamping circuit constructed by switching elements and a clamping capacitor is connected to the current type switching circuit. The converter has a control circuit for cooperatively making switching elements operative so as to control a current flowing in a resonance reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/193,244, filed Aug. 18, 2008 and which present application claimspriority from Japanese patent application No. JP 2007-221892 filed onAug. 28, 2007, the contents of which are hereby incorporated byreference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a bi-directional DC-DC converter having aninsulating function and a method for controlling such a converter.

2. Description of the Related Arts

In recent years, hybrid vehicles having high efficiency have been beingspread due to an increase in consciousness of conservation of the globalenvironment. The hybrid automobile has a main battery for driving amotor and a battery for auxiliaries. If an electric power can bemutually supplied between the two batteries of different voltages,design flexibility of a vehicle power supply systems can be increased.

A bi-directional DC-DC converter for bi-directionally convertingelectric powers between two power sources of different voltages has beendisclosed in JP-A-2002-165448. In the bi-directional DC-DC converterdisclosed in Patent Document 1, a voltage type circuit on a high voltageside and a current type circuit on a low voltage side having a chokecoil are connected through a transformer. By making a switching elementof the high voltage side circuit operative, the electric power issupplied from the high voltage side power source to the low voltage sidepower source. By making a switching element of the low voltage sidecircuit operative, the electric power is supplied from the low voltageside power source to the high voltage side power source.

Further, a bi-directional DC-DC converter in which a voltage clampingcircuit including an object connection in series of a switching elementand a capacitor is connected to a low voltage side circuit has beendisclosed in JP-A-2006-187147. In the bi-directional DC-DC converterdisclosed in Patent Document 2, at the time of a step-down operation, aloss caused by a circulating current is reduced by the voltage clampingcircuit. A high efficient and small bi-directional DC-DC converter inwhich, at the time of the step-up/down operations, the generation of asurge voltage in the low voltage side circuit is prevented and awithstanding voltage of the switching element is reduced is provided.

In the bi-directional DC-DC converter in the related art disclosed inJP-A-2002-165448, since not only the efficiency is deteriorated by aloss due to a circulating current at the time of the step-down operationbut also it is necessary to raise a withstanding voltage of theswitching element by surge voltages caused in the low voltage sidecircuit at the time of the step-up/down operations, they become anobstacle to realization of miniaturization and high efficiency.

In the bi-directional DC-DC converter in the related art disclosed inPatent Document 2 (JP-A-2006-187147), when a step-up voltageratio/output electric power is large at the time of the step-upoperation, it is also necessary to raise the withstanding voltage of theswitching element in the low voltage side circuit and it becomes anobstacle to realization of miniaturization and high efficiency.

SUMMARY OF THE INVENTION

It is one of objects of the invention to provide a small and highefficient bi-directional DC-DC converter in which even when a step-upvoltage ratio/output electric power is large at the time of a step-upoperation, a voltage which is applied to a switching element in a lowvoltage side circuit is reduced.

Another object of the invention is to provide a small bi-directionalDC-DC converter in which a large electric power can be outputted even atthe time of a step-down operation.

To accomplish the above object, according to the invention, there isprovided a bi-directional DC-DC converter which comprises a high voltageside switching circuit, connected between a first DC power source and atransformer, for executing an electric power conversion between a directcurrent and an alternating current, a low voltage side switchingcircuit, connected between a second DC power source and the transformer,for executing an electric power conversion between a direct current andan alternating current, and a control circuit for controlling ON/OFF ofswitching elements included in each of the switching circuits and inwhich an electric power is transmitted and received between the firstand second DC power sources, wherein the high voltage side switchingcircuit includes a first vertical arm in which a first upper armswitching element and a first lower arm switching element are seriallyconnected, a second vertical arm in which a second upper arm switchingelement and a second lower arm switching element are serially connected,a first smoothing capacitor connected in parallel to the first andsecond vertical arms and the first DC power source, and a seriesconnector which is connected between a series node of the first upperarm switching element and the first lower arm switching element and aserial node of the second upper arm switching element and the secondlower arm switching element and is constructed by a resonance capacitor,a resonance reactor, and a primary winding of the transformer, the lowvoltage side switching circuit includes a first switching element groupwhich is connected to the second DC power source and a secondary windingof the transformer and includes a plurality of switching elements, asmoothing reactor connected to the first switching element group and/orthe secondary winding of the transformer, a second smoothing capacitorin which one end is connected to one end of each of the switchingelements included in the first switching element group and the other endis connected to one end of the smoothing reactor and which is connectedin parallel to the second DC power source, and a voltage clampingcircuit which is connected to the switching elements included in thefirst switching element group and has a second switching element groupincluding at least one switching element and a clamping capacitor, andthe control circuit includes first switching means for switching theswitching elements in an OFF state in the second switching element groupto ON for a period of time during which both of the first upper armswitching element and the second lower arm switching element are in anON state and second switching means for switching the first lower armswitching element and the second upper arm switching element to ON whilekeeping the switching elements switched to ON by the first switchingmeans in the ON state.

According to the embodiment of the invention, the control circuitfurther includes means for switching the first lower arm switchingelement and the second upper arm switching element to ON while keepingthe switching elements switched to ON by the first switching means inthe ON state and, after a direction of a current flowing in the primarywinding was reversed, switching the switching elements in the ON stateto OFF.

According to the embodiment of the invention, the control circuitfurther includes: third switching means for switching the switchingelements in the OFF state in the second switching element group to ONfor a period of time during which both of the first lower arm switchingelement and the second upper arm switching element are in the ON state;and fourth switching means for switching the first upper arm switchingelement and the second lower arm switching element to ON while keepingthe switching elements switched to ON by the third switching means inthe ON state.

According to the embodiment of the invention, the control circuitfurther includes means for switching the first upper arm switchingelement and the second lower arm switching element to ON while keepingthe switching elements switched to ON by the third switching means inthe ON state and, after a direction of a current flowing in the primarywinding was reversed, switching the switching elements in the ON stateto OFF.

According to the embodiment of the invention, a voltage which issubstantially twice as large as the first DC power source is applied tothe series connector of the resonance reactor and the resonancecapacitor for a period of time during which the switching elementsswitched to ON by the first or third switching means are in the ONstate.

According to the invention, there is provided a bi-directional DC-DCconverter which comprises a high voltage side switching circuit,connected between a first DC power source and a transformer, forexecuting an electric power conversion between a direct current and analternating current, a low voltage side switching circuit, connectedbetween a second DC power source and the transformer, for executing anelectric power conversion between a direct current and an alternatingcurrent, and a control circuit for controlling ON/OFF of switchingelements included in each of the switching circuits and in which anelectric power is transmitted and received between the first and secondDC power sources, wherein the high voltage side switching circuitincludes a first vertical arm in which a first upper arm switchingelement and a first lower arm switching element are serially connected,a second vertical arm in which a second upper arm switching element anda second lower arm switching element are serially connected, a firstsmoothing capacitor connected in parallel to the first and secondvertical arms and the first DC power source, and a series connectorwhich is connected between a serial node of the first upper armswitching element and the first lower arm switching element and a serialnode of the second upper arm switching element and the second lower armswitching element and is constructed by a resonance capacitor, aresonance reactor, and a primary winding of the transformer, the lowvoltage side switching circuit includes a first switching element groupwhich is connected to the second DC power source and a secondary windingof the transformer and includes a plurality of switching elements, asmoothing reactor connected to the first switching element group and/orthe secondary winding of the transformer, a second smoothing capacitorin which one end is connected to one end of each of the switchingelements included in the first switching element group and the other endis connected to one end of the smoothing reactor and which is connectedin parallel to the second DC power source, and a voltage clampingcircuit which is connected to the switching elements included in thefirst switching element group and has a second switching element groupincluding at least one switching element and a clamping capacitor, andthe control circuit includes means for switching one of the first andsecond upper or lower arm switching elements for a period of time duringwhich an energy is supplied to the first DC power source and controlmeans for controlling timing for switching from a period of time duringwhich an energy is accumulated from the second DC power source into thesmoothing reactor to a period of time during which the energy is emittedand timing for switching the upper or lower arm switching element in theON state to OFF in accordance with the energy which is supplied to thefirst DC power source.

According to the embodiment of the invention, the control means switchesthe upper or lower arm switching element in the ON state to OFF and,after a direction of a current flowing in the primary winding wasreversed, switches the switching elements in the ON state in the firstswitching element group to OFF, and switches a state of the energy inthe smoothing reactor from the accumulation to the emission.

According to the embodiment of the invention, the control means controlsa time which is necessary until the upper or lower arm switching elementin the ON state is switched to OFF and the energy is supplied to thefirst DC power source after the switching elements in the ON state inthe first switching element group were switched to OFF, therebyadjusting an amount of energy which is supplied to the first DC powersource.

According to the embodiment of the invention, the control circuitfurther includes: means for switching two of the first and second upperor lower arm switching elements to ON for the period of time duringwhich the energy is supplied to the first DC power source; means forswitching one of the upper and lower arm switching elements in the ONstate to OFF after a direction of a current flowing in the primarywinding was reversed; and means for switching the upper or lower armswitching elements in the ON state to OFF after the switching elementsin the ON state in the first switching element group were switched toOFF and a state of the energy in the smoothing reactor was switched fromthe accumulation to the emission.

According to the embodiment of the invention, the first switchingelement group includes fifth to eighth switching elements, the secondswitching element group includes a ninth switching element, the voltageclamping circuit includes a series connector of the ninth switchingelement and the clamping capacitor, and the low voltage side switchingcircuit includes a third vertical arm in which the fifth and sixthswitching elements are serially connected and a fourth vertical arm inwhich the seventh and eighth switching elements are serially connectedand is constructed in such a manner that the secondary winding isconnected between a serial node of the fifth and sixth switchingelements and a serial node of the seventh and eighth switching elements,the third and fourth vertical arms and the voltage clamping circuit areconnected in parallel, one end of the smoothing reactor is connected toone end of the voltage clamping circuit, one end of the second smoothingcapacitor is connected to the other end of the smoothing reactor, andthe other end of the voltage clamping circuit is connected to the otherend of the second smoothing capacitor.

According to the embodiment of the invention, the first switchingelement group includes fifth and sixth switching elements, the secondswitching element group includes seventh and eighth switching elements,the voltage clamping circuit is constructed by connecting one end ofeach of the seventh and eighth switching elements and one end of theclamping capacitor, the secondary winding has a connector of one end ofa first secondary winding and one end of a second secondary winding, andthe low voltage side switching circuit is constructed in such a mannerthat one end of the fifth switching element and the other end of theseventh switching element are connected to the other end of the firstsecondary winding, one end of the sixth switching element and the otherend of the eighth switching element are connected to the other end ofthe second secondary winding, the other end of the fifth switchingelement and the other end of the sixth switching element are connectedto the other end of the clamping capacitor, and a series connector ofthe smoothing reactor and the second smoothing capacitor is connectedbetween a node of the fifth and sixth switching elements and a node ofthe first and second secondary windings.

According to the embodiment of the invention, the first switchingelement group includes fifth and sixth switching elements, the secondswitching element group includes seventh and eighth switching elements,the voltage clamping circuit is constructed by connecting one end of theseventh switching element, one end of the eighth switching element, andone end of the clamping capacitor, the smoothing reactor is constructedby connecting one end of a first smoothing reactor and one end of asecond smoothing reactor, and the low voltage side switching circuit isconstructed in such a manner that one end of the fifth switchingelement, the other end of the seventh switching element, and the otherend of the first smoothing reactor are connected to one end of thesecondary winding, one end of the sixth switching element, the other endof the eighth switching element, and the other end of the secondsmoothing reactor are connected to the other end of the secondarywinding of the transformer, the other end of the fifth switching elementand the other end of the sixth switching element are connected to theother end of the clamping capacitor, and the second smoothing capacitoris connected between a node of the fifth and sixth switching elementsand a node of the first and second smoothing reactors.

According to the embodiment of the invention, the clamping capacitor andthe second smoothing capacitor are connected.

According to the embodiment of the invention, the resonance reactor hasa leakage inductance and a wiring inductance of the transformer andincludes first, second, and third resonance reactors which arerespectively serially connected to the primary winding and the secondarywinding and are magnetically coupled therewith, and the resonancecapacitor includes first, second, and third resonance capacitors whichare respectively serially connected to the primary winding and thesecondary winding.

According to the embodiment of the invention, each of the switchingelements has diodes which are connected in inverse parallel and snubbercapacitors which are connected in parallel.

According to the invention, there is provided a control method ofcontrolling a DC-DC converter which comprises a high voltage sideswitching circuit, connected between a first DC power source and atransformer, for executing an electric power conversion between a directcurrent and an alternating current, a low voltage side switchingcircuit, connected between a second DC power source and the transformer,for executing an electric power conversion between a direct current andan alternating current, and a control circuit for controlling ON/OFF ofswitching elements included in each of the switching circuits and inwhich the high voltage side switching circuit includes a first verticalarm in which a first upper arm switching element and a first lower armswitching element are serially connected, a second vertical arm in whicha second upper arm switching element and a second lower arm switchingelement are serially connected, a first smoothing capacitor connected inparallel to the first and second vertical arms and the first DC powersource, and a series connector which is connected between a serial nodeof the first upper arm switching element and the first lower armswitching element and a serial node of the second upper arm switchingelement and the second lower arm switching element and is constructed bya resonance capacitor, a resonance reactor, and a primary winding of thetransformer, and the low voltage side switching circuit includes a firstswitching element group which is connected to the second DC power sourceand a secondary winding of the transformer and includes a plurality ofswitching elements, a smoothing reactor connected to the first switchingelement group and/or the secondary winding of the transformer, a secondsmoothing capacitor in which one end is connected to one end of each ofthe switching elements included in the first switching element group andthe other end is connected to one end of the smoothing reactor and whichis connected in parallel to the second DC power source, and a voltageclamping circuit which is connected to the switching elements includedin the first switching element group and has a second switching elementgroup including at least one switching element and a clamping capacitor,comprising the steps of: switching the switching elements in an OFFstate in the second switching element group to ON for a period of timeduring which both of the first upper arm switching element and thesecond lower arm switching element are in an ON state; and switching thefirst lower arm switching element and the second upper arm switchingelement to ON while keeping the switching elements switched to ON in theON state.

According to the invention, there is provided a control method ofcontrolling a DC-DC converter which comprises a high voltage sideswitching circuit, connected between a first DC power source and atransformer, for executing an electric power conversion between a directcurrent and an alternating current, a low voltage side switchingcircuit, connected between a second DC power source and the transformer,for executing an electric power conversion between a direct current andan alternating current, and a control circuit for controlling ON/OFF ofswitching elements included in each of the switching circuits and inwhich the high voltage side switching circuit includes a first verticalarm in which a first upper arm switching element and a first lower armswitching element are serially connected, a second vertical arm in whicha second upper arm switching element and a second lower arm switchingelement are serially connected, a first smoothing capacitor connected inparallel to the first and second vertical arms and the first DC powersource, and a series connector which is connected between a serial nodeof the first upper arm switching element and the first lower armswitching element and a serial node of the second upper arm switchingelement and the second lower arm switching element and is constructed bya resonance capacitor, a resonance reactor, and a primary winding of thetransformer, and the low voltage side switching circuit includes a firstswitching element group which is connected to the second DC power sourceand a secondary winding of the transformer and includes a plurality ofswitching elements, a smoothing reactor connected to the first switchingelement group and/or the secondary winding of the transformer, a secondsmoothing capacitor in which one end is connected to one end of each ofthe switching elements included in the first switching element group andthe other end is connected to one end of the smoothing reactor and whichis connected in parallel to the second DC power source, and a voltageclamping circuit which is connected to the switching elements includedin the first switching element group and has a second switching elementgroup including at least one switching element and a clamping capacitor,comprising: a first step of switching one of the first and second upperor lower arm switching elements for a period of time during which anenergy is supplied to the first DC power source; a second step ofswitching from a period of time during which an energy is accumulatedfrom the second DC power source into the smoothing reactor to a periodof time during which the energy is emitted; and a third step ofswitching the upper or lower arm switching element in an ON state toOFF, wherein timing for the second step and timing for the third stepare controlled in accordance with the energy which is supplied to thefirst DC power source.

In the control method for the bi-directional DC-DC converter accordingto the invention, the control processes of the control circuit includedin the bi-directional DC-DC converter according to any one of the aboveembodiments are selectively switched in accordance with a propagatingdirection of the energy, an input voltage, an input current, an outputvoltage, and an output current.

According to the preferred embodiments of the invention, the small andhigh efficient bi-directional DC-DC converter constructed in such amanner that even when the step-up voltage ratio/output electric power islarge at the time of the step-up operation, the voltage which is appliedto the switching element of the low voltage side circuit is reduced canbe provided.

According to the preferred embodiments of the invention, the small andhigh efficient bi-directional DC-DC converter which can output the largeelectric power even at the time of the step-down operation can beprovided.

Other objects and features of the present invention will be apparentfrom the following description of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit constructional diagram of a bi-directional DC-DCconverter according to an embodiment of the invention;

FIG. 2 is a voltage/current waveform diagram for describing thestep-down operation 1 of the bi-directional DC-DC converter according tothe embodiment of the invention;

FIG. 3 is a voltage/current waveform diagram for describing thestep-down operation 2 of the bi-directional DC-DC converter according tothe embodiment of the invention;

FIG. 4 is a voltage/current waveform diagram for describing the step-upoperation 1 of the bi-directional DC-DC converter according to theembodiment of the invention;

FIG. 5 is a voltage/current waveform diagram for describing the step-upoperation 2 of the bi-directional DC-DC converter according to theembodiment of the invention; and

FIG. 6 is a voltage/current waveform diagram for describing the step-upoperation 3 of the bi-directional DC-DC converter according to theembodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the invention will be described in detail withreference to the drawings. The same or corresponding portions in thedrawings are designated by the same reference numerals. In theembodiment, although it is assumed that an IGBT and a MOSFET are used asswitching elements as an example, the invention is not limited to them.A voltage of the switching element in an ON state or a voltage which isalmost equal to or lower than a forward drop voltage of a diode isassumed to be a zero voltage.

FIG. 1 is a circuit constructional diagram of a bi-directional DC-DCconverter 5 according to the embodiment of the invention. In FIG. 1, asmoothing capacitor 12, a load 14, a first switching arm in which anemitter of an IGBT 101 and a collector of an IGBT 102 are connected, anda second switching arm in which an emitter of an IGBT 103 and acollector of an IGBT 104 are connected are connected in parallel to apower source 10 on a high voltage side.

Each of diodes 111 to 114 is connected between a collector and anemitter of each of the IGBTs 101 to 104 so as to allow a current to flowfrom the emitter side to the collector side. In the case of using theMOSFET in place of the IGBT, body diodes may be used as diodes 111 to114. Each of snubber capacitors 121 to 124 is connected between thecollector and the emitter of each of the IGBTs 101 to 104.

A primary winding 31 of a transformer 30, a resonance reactor 20, and aresonance capacitor 22 are serially connected between a node of theIGBTs 101 and 102 and a node of the IGBTs 103 and 104. The resonancereactor 20 may be replaced by a leakage inductance and a wiringinductance of the transformer 30 here.

A smoothing capacitor 42 and a load 44 are connected in parallel to apower source 40 on the low voltage side. One end of a secondary winding32 of the transformer 30, one end of a secondary winding 33 of thetransformer 30, and one end of a smoothing reactor 46 are connected. Theother end of the smoothing reactor 46 is connected to a positivepolarity of the power source 40. The other end of the secondary winding32 is connected to a drain of a MOSFET 201. The other end of thesecondary winding 33 is connected to a drain of a MOSFET 202. A sourceof the MOSFET 201 and a source of the MOSFET 202 are connected to anegative polarity of the power source 40.

In a voltage clamping circuit in which a drain of a MOSFET 203, a drainof a MOSFET 204, and one end of a clamping capacitor 48 are connected, asource of the MOSFET 203 is connected to the drain of the MOSFET 201, asource of the MOSFET 204 is connected to the drain of the MOSFET 202,and the other end of the clamping capacitor 48 is connected to thenegative polarity of the power source 40.

Each of diodes 211 to 214 is connected between the drain and the sourceof each of the MOSFETs 201 to 204 so as to allow a current to flow fromthe source side to the drain side. Body diodes of MOSFETs can be alsoused as diodes 211 to 214. Each snubber capacitors may be connectedbetween the drain and the source of each of the MOSFETs 201 to 204.

The IGBTs 101 to 104 and MOSFETs 201 to 204 are switching-controlled bya control circuit 100. Voltage sensors 51 to 54 and current sensors 61to 63 are connected to the control circuit 100.

Prior to describing the operation in detail, voltages and currents inthe circuit diagram of FIG. 1 will be defined. First, as forcollector-emitter voltages V(101) to V(104) of the IGBTs 101 to 104, thecollector is set to be positive and, as for gate-source voltages Vg(101)to Vg(104), the gate is set to be positive. Synthetic currents flowingin the IGBTs 101 to 104 and the diodes 111 to 114 connected in paralleltherewith are respectively assumed to be I(101) to I(104) when adirection of the current flowing from the collector to the emitter ofeach of the IGBTs 101 to 104 is set to be positive.

As for drain-source voltages V(201) to V(204) of the MOSFETs 201 to 204,the drain is set to be positive and, as for gate-source voltages Vg(201)to Vg(204), the gate is set to be positive. Synthetic currents flowingin the MOSFETs 201 to 204 and the diodes 211 to 214 connected inparallel therewith are respectively assumed to be I(201) to I(204) whena direction of the current flowing from the drain to the source of eachof the MOSFETs 201 to 204 is set to be positive.

As for a voltage V(22) of the resonance capacitor 22, a voltage V(20) ofthe resonance reactor 20, and a voltage V(31) of the primary winding 31,a direction of the voltage applied from the second switching arm to thefirst switching arm is set to be positive. As for a current I(20)flowing in the resonance reactor 20, a direction of the current flowingfrom the first switching arm to the second switching arm is set to bepositive.

As for a current I(48) flowing in the clamping capacitor 48, a directionof the current flowing from a node of the MOSFETs 203 and 204 to thenegative polarity of the power source 40 is set to be positive. Avoltage of the drain of each of the MOSFETs 203 and 204 in the case ofsetting the negative polarity of the power source 40 to a reference isassumed to be a voltage V(48) of the clamping capacitor 48.

A direction of a current I(46) flowing in the smoothing reactor 46 isdefined as follows. In the case of the step-down operation for feedingthe energy of the power source 10 to the power source 40, a direction ofthe voltage applied from a node of the secondary winding 32 and thesecondary winding 33 to the positive polarity of the power source 40 isset to be positive. In the case of the step-up operation for feeding theenergy of the power source 40 to the power source 10, a direction of thevoltage applied from the positive polarity of the power source 40 to thenode of the secondary winding 32 and the secondary winding 33 is set tobe positive. As for a direction of a voltage V(46) of the smoothingreactor 46, in the case of each of the step-down operation and thestep-up operation, a direction adapted to accelerate the current I(46)in the smoothing reactor 46 in the positive direction is set to bepositive.

The operation of the bi-directional DC-DC converter 5 according to theembodiment of the invention will be described in detail hereinbelow withreference to the drawings. In this instance, the operation for feedingthe energy of the power source 10 to the power source 40 is assumed tobe the step-down operation and the operation for feeding the energy ofthe power source 40 to the power source 10 is assumed to be the step-upoperation.

First, the step-down operation which is executed by the bi-directionalDC-DC converter 5 will be described.

[Step-Down Operation 1]

FIG. 2 is a voltage/current waveform diagram for describing thestep-down operation 1. The step-down operation 1 will be explained indetail hereinbelow with reference to FIG. 2. In FIG. 2, a1 to h1 denoteperiods of time.

(Period of Time a1)

First, for the period of time a1, the IGBTs 101 and 104 are in the ONstate, the IGBTs 102 and 103 are in the OFF state, and the voltage ofthe power source 10 is applied to primary winding 31 of the transformer30 through the IGBTs 101 and 104, resonance capacitor 22, and resonancereactor 20. The MOSFETs 202 and 203 are in the OFF state, the voltagedeveloped in the secondary winding 32 is applied to the smoothingreactor 46 through the power source 40 and diode 211, the current I(46)increases gradually, and the energy is supplied to the power source 40.The voltages developed in the secondary windings 32 and 33 are appliedto the clamping capacitor 48 through the diodes 214 and 211, so that theclamping capacitor 48 is charged.

Although the currents I(201) and I(204) are negative, assuming that theMOSFETs 201 and 204 are in the ON state at this time, by shunting thecurrents flowing in the diodes 211 and 214 to the MOSFETs 201 and 204,the loss can be reduced. Such an operation that when a forward currentof the diode flows in the diode which has been connected in inverseparallel with the MOSFET or the body diode of the MOSFET, this MOSFET isset to ON and the loss is reduced as mentioned above is hereinbelowreferred to as a synchronous rectification. At this time, the MOSFET 204is held in the ON state (zero voltage switching).

(Period of time b1)

For the period of time b1, the IGBT 104 is turned off and, thereafter,the IGBT 103 is turned on. When the IGBT 104 is turned off, the currentI(20) which has been flowing in the IGBT 104 discharges the snubbercapacitor 123 while charging the snubber capacitor 124. When the voltageV(103) reaches the zero voltage, the diode 113 is made conductive. Atthis time, the IGBT 103 is turned on (zero voltage switching). Thecurrent I(20) is refluxed by a path passing through the resonancereactor 20, primary winding 31, diode 113, IGBT 101, and resonancecapacitor 22. The current flowing in the primary winding 31 as mentionedabove is hereinbelow referred to as a circulating current.

Since the MOSFET 204 is in the ON state, the current I(201) is negative,and the voltage V(201) is equal to the zero voltage, the voltage V(48)of the clamping capacitor 48 is applied to the secondary windings 32 and33. The voltage developed in the primary winding 31 is applied to theresonance reactor 20 and gradually decreases the current I(20).Therefore, the circulating current decreases and the energy which islost on the path through which the circulating current flows can bereduced. In association with the decrease in circulating current, adischarge current of the clamping capacitor 48 increases. At this time,since the voltage has been developed in the secondary winding 32, in amanner similar to the period of time a1, the voltage is applied to thesmoothing reactor 46, the current I(46) increases gradually, and theenergy is supplied to the power source 40.

(Period of Time c1)

For the period of time c1, the MOSFET 204 is turned off and, thereafter,the MOSFET 202 is turned on. When the MOSFET 204 is turned off, thedischarge of the clamping capacitor 48 is finished and the decrease incirculating current also becomes gentle. However, since charges havebeen accumulated in the resonance capacitor 22 and the voltage has beendeveloped in the direction adapted to reduce the circulating current,the circulating current decreases gently. The current which has beenflowing in the MOSFET 204 is commutated to the diode 212. At this time,by turning on the MOSFET 202, the synchronous rectification isperformed.

The energy accumulated in the smoothing reactor 46 is supplied to thepower source 40 and the current I(46) decreases gradually. Although thecurrent I(46) had been flowing on the path passing through the diode 211(MOSFET 201) and secondary winding 32 for the period of time a1, sincethe circulating current has been reduced for the period of time b1, thecurrent is also shunted to a path passing through the diode 212 (MOSFET202) and secondary winding 33 for the period of time c1. The more thecirculating current is reduced, the more the current is equivalentlyshunted to those two paths. Consequently, the conduction loss can bereduced.

The more the circulating current is reduced, the more the conductionloss can be reduced. However, for a period of time d1, which will bedescribed hereinafter, the circulating current of a certain amount hasto be left in order to enable the IGBT 102 to perform the zero voltageswitching. A circulating current value necessary to enable the IGBT 102to perform the zero voltage switching can be arithmetically operatedfrom a voltage of the power source 10 (voltage sensor 51), anelectrostatic capacitance of the resonance capacitor 22, an inductanceof the resonance reactor 20, electrostatic capacitances of the snubbercapacitors 121 to 124, and a dead time of the IGBTs 101 and 102 (or adead time of the IGBTs 103 and 104).

In order to control the circulating current so as to have thecirculating current value obtained by the above arithmetical operation,off timing for the MOSFET 204 has to be accurately decided. Such timingcan be arithmetically operated on the basis of an off time lag of theIGBT 104 and the MOSFET 204 from the voltage of the power source 40(voltage sensor 52), the current I(46) in the smoothing reactor 46(current sensor 62), and a turn ratio of the transformer 30 in additionto the information which has been used in order to obtain thecirculating current value necessary to enable the IGBT 102 to performthe zero voltage switching in the above process, or the off timing forthe MOSFET 204 may be determined on the basis of a measurement valuefrom the current sensor 61 showing the input current of a full bridgeand a measurement value from the current sensor 63 showing thecirculating current.

(Period of Time d1)

For the period of time d1, the IGBT 101 is turned off and, thereafter,the IGBT 102 is turned on and the MOSFET 201 is turned off. When theIGBT 101 is turned off, the circulating current which has been flowingin the IGBT 101 discharges the snubber capacitor 122 while charging thesnubber capacitor 121. When the voltage V(102) reaches the zero voltage,the diode 112 is made conductive. At this time, the IGBT 102 is turnedon (zero voltage switching). The MOSFET 201 is turned off before theperiod of time d1 is finished. The circulating current flows in thediode 112, resonance capacitor 22, resonance reactor 20, primary winding31, and diode 113 and reaches the power source 10. The voltage of thepower source 10 is applied to the resonance reactor 20 and thecirculating current decreases.

Since the IGBTs 102 and 103 are in the ON state, after the circulatingcurrent reached zero, the circulating current increases in the reversedirection. In association with it, the current flowing through the diode211 (MOSFET 201) and the secondary winding 32 decreases and the currentflowing through the diode 212 (MOSFET 202) and the secondary winding 33increases. The MOSFET 201 is turned off before the current flowingthrough the secondary winding 32 reaches zero (the current flowingthrough the secondary winding 33 reaches the current I(46)).

(Period of Time e1)

For the period of time e1, the MOSFET 203 is turned on. When the currentflowing through the secondary winding 32 reaches zero, the diode 211 isreversely made conductive and, thereafter, reversely recovered. Thecurrent which had been flowing during the reverse conduction iscommutated to the diode 213 after the diode 211 was reversely recovered.At this time, the MOSFET 203 is turned on (zero voltage switching).

When the diode 211 is reversely recovered, the drain voltage of theMOSFET 201 rises. Therefore, the MOSFET 203 can be also turned on bydetecting such a voltage increase by the voltage sensor 53.

In the converter circuit in the related art having no voltage clampingcircuit, when the diode 211 is reversely recovered, there is a casewhere a surge voltage is generated in the drain voltage of the MOSFET201. In the embodiment, however, the clamping capacitor 48 suppressesthe generation of the surge voltage.

The voltage of the power source 10 has been applied to the primarywinding 31 of the transformer 30 through the IGBTs 102 and 103,resonance capacitor 22, and resonance reactor 20. The MOSFETs 201 and204 are in the OFF state, the voltage developed in the secondary winding33 is applied to the smoothing reactor 46 through the power source 40and diode 212, the current I(46) increases gradually, and the energy issupplied to the power source 40. The voltages developed in the secondarywindings 32 and 33 are applied to the clamping capacitor 48 through thediodes 213 and 212, thereby charging the clamping capacitor 48.

The operation for the period of time e1 is symmetrical with theoperation for the period of time a1. Subsequently, after the periods oftime f1 to h1, the operation cycle is returned to the period of time a1.Since the operations for the periods of time f1 to h1 are symmetricalwith those for the periods of time b1 to d1, their detailed descriptionis omitted here.

Since the voltage is developed in the resonance capacitor 22 in thedirection adapted to raise the voltage which is applied to the primarywinding 31 for the periods of time a1 and e1 mentioned above, theresonance capacitor 22 provides an effect of increasing the outputelectric power.

When the diodes 211 and 212 are reversely recovered, the voltageclamping circuit suppresses the generation of the surge voltage asmentioned above. Therefore, the voltage clamping circuit has such aneffect that elements of a low withstanding voltage can be used as diodes211 and 212 and MOSFETs 201 and 202.

Since the circulating current exists, the zero voltage switching at thetime of turning on the IGBTs 101 and 102 can be realized as mentionedabove. Therefore, since the circulating current decreases at the time ofthe small load, it is difficult to perform the zero voltage switching.To solve such a problem, for example, if the MOSFET 204 is turned offand the MOSFET 204 is turned on for the periods of time b1 and c1 priorto turning off the IGBT 104, since the circulating current increases,the zero voltage switching at the time of turning on the IGBT 101 can berealized even in the case of the small load. Order of the timing forturning on the MOSFET 202 and the timing for turning off the IGBT 104 isnot limited. If the MOSFET 204 is turned off, since its breaking currentis commutated to the diode 212, the zero voltage switching of theturn-on of the MOSFET 202 can be also performed. This is true of theperiods of time f1 and g1. Therefore, the above operating method has aneffect of improving the efficiency even at the time of the small load.

In the step-down operation 1 described above, a time-dependent ratio ofthe period of time during which the IGBTs 101 and 104 are simultaneouslyin the ON state and a time-dependent ratio of the period of time duringwhich the IGBTs 102 and 103 are simultaneously in the ON state arechanged, thereby adjusting the output electric power. Such atime-dependent ratio is called a duty. The larger the duty is, thelarger electric power can be outputted. Therefore, in the step-downoperation 1, when the ON/OFF states of the IGBTs 101 and 104 coincideand the ON/OFF states of the IGBTs 102 and 103 coincide, that is, whenthe duty is maximum, the output electric power becomes maximum. In thecase of setting the output electric power to be further larger than thatin the above state, the step-down operation 2, which will be describedhereinbelow is applied.

[Step-Down Operation 2]

FIG. 3 is a voltage/current waveform diagram for describing thestep-down operation 2. The step-down operation 2 will be described indetail hereinbelow with reference to FIG. 3. In FIG. 3, a2 to f2correspond to the periods of time a2 to f2.

(Period of Time a2)

The operation for the period of time a2 is similar to that for theperiod of time a1 of the step-down operation 1 mentioned above and itsdetailed explanation is omitted here.

(Period of Time b2)

As mentioned above, the ON/OFF states of the IGBTs 101 and 104 coincidein the step-down operation 2. When the IGBTs 101 and 104 are turned off,the current I(20) discharges the snubber capacitor 122 while chargingthe snubber capacitor 121 and discharges the snubber capacitor 123 whilecharging the snubber capacitor 124. When the voltage V(102) reaches thezero voltage, the diode 112 is made conductive. When the voltage V(103)reaches the zero voltage, the diode 113 is made conductive. At thistime, the IGBTs 102 and 103 are turned on (zero voltage switching).

The current I(20) flows through the diode 112, resonance capacitor 22,resonance reactor 20, primary winding 31, and diode 113 and reaches thepower source 10. Since the MOSFET 204 is in the ON state, the currentI(20) is negative, and voltage V(201) is equal to the zero voltage, thevoltage V(48) in the clamping capacitor 48 is applied to the secondarywindings 32 and 33. The voltage of the power source 10 is applied to theresonance reactor 20 and the current I(20) decreases. At this time,since the voltage developed in the primary winding 31 is alsoadditionally applied to the resonance reactor 20, a decreasing speed ofthe current I(20) is higher than that for each of the periods of time b1and d1 in the step-down operation 1. The voltage developed in theprimary winding 31 is almost equal to the voltage of the power source 10here. The voltage has also been developed in the resonance capacitor 22in the same direction as that of the voltage developed in the primarywinding 31. The voltage which is twice or more times as high as thevoltage of the power source 10 is applied to the resonance reactor 20.

Since the IGBTs 102 and 103 are in the ON state, after the current I(20)reached the zero voltage, a magnitude of the current I(20) increases inthe reverse direction. At this time, Since the MOSFET 204 is in the ONstate and the voltage have been developed in the secondary winding 32,in a manner similar to the period of time a2, the voltage is applied tothe smoothing reactor 46, the current I(46) increases gradually, and theenergy is supplied to the power source 40.

(Period of Time c2)

When the MOSFET 204 is turned off, the discharge of the clampingcapacitor 48 is finished and the voltage in the primary winding 31 isnot additionally applied to the resonance reactor 20. Therefore, theincrease in magnitude of the current I(20) becomes gentle. The currentwhich has been flowing in the MOSFET 204 is commutated to the diode 212.At this time, by turning on the MOSFET 202, the synchronousrectification is performed.

The energy accumulated in the smoothing reactor 46 is supplied to thepower source 40 and the current I(46) decreases gradually.

The current flowing in the diode 211 (MOSFET 201) and the secondarywinding 32 decreases and the current flowing in the diode 212 (MOSFET202) and the secondary winding 33 increases. The MOSFET 201 is turnedoff before the current flowing in the secondary winding 32 reaches zero(the current flowing in the secondary winding 33 reaches the currentI(46)).

(Period of Time d2)

The operation for the period of time d2 is similar to that for theperiod of time e1 of the step-down operation 1 and its detailedexplanation is omitted here. The operation for the period of time d2 issymmetrical with that for the period of time a2. Subsequently, after theperiods of time e2 and f2, the operation cycle is returned to the periodof time a2. Since the operations for the periods of time e2 and f2 aresymmetrical with those for the periods of time b2 and c2, their detaileddescription is omitted here.

Also in the step-down operation 2, in a manner similar to the step-downoperation 1, the resonance capacitor 22 has an effect of increasing theoutput electric power. The voltage clamping circuit has such an effectthat elements of a low withstanding voltage can be used as diodes 211and 212 and MOSFETs 201 and 202.

In the step-down operation 2, it is a feature that not only for theperiods of time a2 and d2 but also for the periods of time b2 and e2,the positive voltage is applied to the smoothing reactor 46 and thevoltage which is twice or more times as high as the voltage of the powersource 10 is applied to the resonance reactor 20 and a change ratio ofthe current I(20) is increased, thereby extending the periods of time a2and d2. Particularly, it is a feature that even after the polarity ofthe current I(20) was inverted, the voltage which is twice or more timesas high as the voltage of the power source 10 is applied to theresonance reactor 20. If the resonance capacitor 22 is not provided, thevoltage which is applied to the resonance reactor 20 is equal to avoltage which is about twice as high as the voltage of the power source10. Thus, a point that the electric power larger than the maximum outputelectric power in the step-down operation 1 can be outputted is amaximum advantage.

In the step-down operation 2, the ON/OFF states of the IGBTs 101 and 104coincide, the ON/OFF states of the IGBTs 102 and 103 coincide asmentioned above, and the duty is maximum. Therefore, the output electricpower is adjusted by changing the durations of the periods of time b2and e2, that is, by changing the off timing of the MOSFETs 203 and 204.

The later the off timing of the MOSFETs 203 and 204 is delayed, the morethe output electric power increases. The off timing can be delayed to apoint before or after the timing when the diodes 211 and 212 arereversely recovered. The off timing of the MOSFETs 203 and 204 can bealso decided by detection signals from the voltage sensors 53 and 54 fordetecting the reverse recovery of the diodes 211 and 212. In this case,the periods of time c2 and f2 do not exist.

The step-up operation which is executed by the bi-directional DC-DCconverter 5 will be described.

[Step-Up Operation 1]

FIG. 4 is a voltage/current waveform diagram for describing the step-upoperation 1. The step-up operation 1 will be explained in detailhereinbelow with reference to FIG. 4. In FIG. 4, A1 to F1 denote periodsof time.

(Period of Time A1)

First, for the period of time A1, the MOSFETs 201 and 202 are in the ONstate and the MOSFETs 202 and 203 are in the OFF state. The voltage ofthe power source 40 is applied to the smoothing reactor 46 through thesecondary windings 32 and 33 and MOSFETs 201 and 202 and the energy ofthe power source 40 is accumulated to the resonance reactor 46.

The IGBT 103 is in the ON state, the IGBTs 101, 102, and 104 are in theOFF state, and the circulating current flows on a path passing throughthe IGBT 103, primary winding 31, resonance reactor 20, resonancecapacitor 22, and diode 111. Since the charges have been accumulated inthe resonance capacitor 22 and the voltage has been developed in thedirection adapted to increase the circulating current, the circulatingcurrent increases gradually.

(Period of Time B1)

When the MOSFET 202 and IGBT 103 are turned off, the current which hasbeen flowing in the MOSFET 202 flows in the diode 214, thereby chargingthe clamping capacitor 48. At this time, the MOSFET 204 is turned on(zero voltage switching). The circulating current which has been flowingin the IGBT 103 discharges the snubber capacitor 124 while charging thesnubber capacitor 123. When the voltage V(104) reaches the zero voltage,the diode 114 is made conductive. At this time, the IGBT 104 is turnedon (zero voltage switching).

The voltage V(48) of the clamping capacitor 48 is applied to thesecondary windings 32 and 33. The voltage obtained by subtracting thevoltage of the power source 10 from the voltage developed in the primarywinding 31 is applied to the resonance reactor 20 and the magnitude ofthe current I(20) increases. The current I(20) flows through the diode114, primary winding 31, resonance reactor 20, resonance capacitor 22,and diode 111 and reaches the power source 10. The energy is supplied tothe power source 10. The energy accumulated in the smoothing reactor 46is emitted and the current I(46) decreases.

In association with an increase in magnitude of the current I(20), thecharge current of the clamping capacitor 48 decreases and the dischargeis performed soon.

(Period of Time C1)

When the MOSFET 204 is turned off, the discharge current of the clampingcapacitor 48 which has been flowing in the MOSFET 204 makes the diode212 conductive. At this time, the MOSFET 202 is turned on (zero voltageswitching).

Since the voltage V(48) of the clamping capacitor 48 is not applied tothe secondary windings 32 and 33, no voltage is developed in the primarywinding 31. The voltage of the power source 10 is applied to theresonance reactor 20 and the magnitude of the current I(20) decreases.In association with it, the direction of the current I(202) is changedfrom the negative to the positive.

In a manner similar to the period of time A1, the voltage of the powersource 40 is applied to the smoothing reactor 46 and the energy of thepower source 40 is accumulated in the smoothing reactor 46.

(Period of Time D1)

Since the IGBT 104 is in the ON state and the IGBT 101 is in the OFFstate, when the current I(20) reaches zero, first, the diode 111 isreversely made conductive. After that, when the diode is reverselyrecovered, a snubber capacitor C102 is discharged while the snubbercapacitor 121 is charged. When the voltage V(102) reaches the zerovoltage, the diode 112 is made conductive. For a period of time untilthe diode 112 is made conductive after the diode 111 was reversely madeconductive, the energy of the power source 10 accumulated in theresonance reactor 20 becomes the circulating current flowing on the pathpassing through the diode 112, resonance capacitor 22, resonance reactor20, primary winding 31, and IGBT 104. Since the charges have beenaccumulated in the resonance capacitor 22 and the voltage has beendeveloped in the direction adapted to increase the circulating current,the circulating current increases gradually.

In a manner similar to the period of time A1, the voltage of the powersource 40 is applied to the smoothing reactor 46 and the energy of thepower source 40 is accumulated in the smoothing reactor 46.

The operation for the period of time D1 is symmetrical with that for theperiod of time A1. Subsequently, after the periods of time E1 and F1,the operation cycle is returned to the period of time A1. Since theoperations for the periods of time E1 and F1 are symmetrical with thosefor the periods of time B1 and C1, their detailed description is omittedhere.

By turning on the IGBT 101 for the periods of time A1 to C1 and turningon the IGBT 102 for the periods of time D1 to F1, the synchronousrectification is performed.

The direction of the circulating current flowing when the MOSFET 202 isturned off for the period of time B1 is equal to the direction of theenergy which is sent to the power source 10 for the periods of time B1and C1. Therefore, the larger the circulating current is, the largeroutput electric power is liable to be obtained. The current I(202) atthe time of turning off the MOSFET 202 decreases and the energy which islost when the MOSFET 202 is turned off can be reduced. However, byshutting off the current I(202), the energy is accumulated in theclamping capacitor. When the MOSFET 202 is turned on for the period oftime C1, the zero voltage switching can be performed by using such anenergy. Therefore, the larger the circulating current is, the more it isdifficult to perform the zero voltage switching at the time of turningon the MOSFET 202. This is true of the period of time E1.

Therefore, in the case of the small load, since the current to be shutoff is small, the zero voltage switching at the time of turning on theMOSFET 202 becomes difficult. To solve such a problem, for example, ifthe IGBT 103 is turned off for the period of time B1 prior to turningoff the MOSFET 202, since the circulating current is reduced orreversely flows, the current to be shut off at the time of turning offthe MOSFET 202 increases. Even in the case of the small load, the zerovoltage switching can be realized when the MOSFET 202 is turned on. Thisis true of the period of time E1. Thus, the above operating method hasan effect of raising the efficiency even at the time of the small load.

On the contrary, if the IGBT 103 is turned off after the MOSFET 202 wasturned off, the large output electric power can be obtained. This istrue of the period of time E1. The above operation will be described asa step-up operation 2 hereinafter.

In the step-up operation 1, the output electric power is adjusted bychanging the time-dependent ratio (on duty) of the ON periods of time ofthe MOSFETs 201 and 202. If the on duty is increased, the voltage V(48)of the clamping capacitor 48 rises and the voltage developed in theprimary winding 31 also rises, so that the output electric powerincreases. However, the voltages which are applied to the clampingcapacitor 48, MOSFETs 201 to 204, and diodes 211 to 214 are determinedby the voltage of the power source 40, the on duty, the wiringinductance, and the breaking currents of the MOSFETs 201 to 204.Therefore, in order to suppress the voltages which are applied to thoseelements, there is an upper limitation in the increase in on duty. Ifthe output electric power is further increased in site of the fact thatthe on duty is equal to the upper limit, the step-up operation 2, whichwill be described hereinbelow, is applied.

[Step-Up Operation 2]

FIG. 5 is a voltage/current waveform diagram for describing the step-upoperation 2. The step-up operation 2 will be explained in detailhereinbelow with reference to FIG. 5. In FIG. 5, A2 to H2 denote periodsof time.

(Period of Time A2)

The operation for the period of time A2 is similar to that for theperiod of time A1 of the step-up operation 1 and its detailedexplanation is omitted here.

(Period of Time B2)

When the MOSFET 202 is turned off, the current which has been flowing inthe MOSFET 202 flows in the diode 214 and charges the clamping capacitor48. At this time, the MOSFET 204 is turned on (zero voltage switching).

Although the voltage is developed in the primary winding 31 in a mannersimilar to that for the period of time B1 of the step-up operation 1,different from the period of time B1 of the step-up operation 1, theIGBT 103 is in the ON state. Therefore, since the voltage obtainedwithout subtracting the voltage of the power source 10 from the voltagedeveloped in the primary winding 31 is applied to the resonance reactor20, the magnitude of the current I(20) increases at a speed higher thanthat for the period of time B1 of the step-up operation 1. At this time,the current I(20) flows on the same path of the circulating current asthat for the period of time A2.

The energy accumulated in the smoothing reactor 46 is emitted and thecurrent I(46) decreases.

(Period of Time C2)

When the IGBT 103 is turned off, the circulating current which has beenflowing in the IGBT 103 discharges the snubber capacitor 124 whilecharging a snubber capacitor C103. When the voltage V(104) reaches thezero voltage, the diode 114 is made conductive. At this time, the IGBT104 is turned on (zero voltage switching).

The voltage has been developed in the primary winding 31 in a mannersimilar to that for the period of time B2. The current I(20) flowsthrough the diode 114, primary winding 31, resonance reactor 20,resonance capacitor 22, and diode 111 and reaches the power source 10.The energy is supplied to the power source 10. The energy accumulated inthe smoothing reactor 46 is emitted and the current I(46) decreases.

In association with the increase in magnitude of the current I(20), thecharge current of the clamping capacitor 48 decreases and the dischargeis performed soon.

(Period of Time D2)

The operation for the period of time D2 is similar to that for theperiod of time C1 of the step-up operation 1 and its detailedexplanation is omitted here.

(Period of Time E2)

The operation for the period of time E2 is similar to that for theperiod of time D1 of the step-up operation 1 and its detailedexplanation is omitted here. The operation for the period of time E2 issymmetrical with that for the period of time A2. Subsequently, after theperiods of time F2 to H2, the operation cycle is returned to the periodof time A2. Since the operations for the periods of time F2 to H2 aresymmetrical with those for the periods of time B2 to D2, their detaileddescription is omitted here.

In the step-up operation 2, the output electric power is adjusted bychanging the off timing of the IGBTs 103 and 104 and by changing theduration of the periods of time B2 and F2. If the off timing of theIGBTs 103 and 104 is delayed, a change ratio of the current I(20) foreach of the periods of time C2 and G2 decreases, a voltage drop due to aresonance reactor L is suppressed, and the output electric powerincreases. However, if the output electric power is increased bydelaying the off timing of the IGBTs 103 and 104, a peak value of thecurrent I(20) increases. The breaking currents of the MOSFETs 201 and202 increase. Surge voltages which cannot be completely suppressed bythe voltage clamping circuit are caused in the drains of the MOSFETs 201and 202. The voltages which are applied to the MOSFETs 201 and 202 rise.There is, consequently, upper limits in the off timing of the IGBTs 103and 104 which can be delayed.

In the case of further increasing the output electric power to a valuelarger than that in the step-up operation 2, it is effective to increasethe circulating current to be previously allowed to flow. A step-upoperation 3, which will be described hereinbelow is applied.

[Step-Up Operation 3]

FIG. 6 is a voltage/current waveform diagram for describing the step-upoperation 3. The step-up operation 3 will be explained in detailhereinbelow with reference to FIG. 6. In FIG. 6, A3 to H3 denote periodsof time.

(Period of Time A3)

Although the operation for the period of time A3 is similar to that forthe period of time A2 of the step-up operation 2 and its detailedexplanation is omitted here, a larger circulating current is flowing.The IGBT 101 is in the ON state.

(Period of Time B3)

The operation for the period of time B3 is similar to that for theperiod of time B2 of the step-up operation 2 and its detailedexplanation is omitted here.

(Period of Time C3)

The operation for the period of time C3 is similar to that for theperiod of time C2 of the step-up operation 2 and its detailedexplanation is omitted here.

(Period of Time D3)

The operation which is executed for a period of time until the magnitudeof the current I(20) decreases and reaches zero is similar to that forthe period of time D2 of the step-up operation 2 and its detailedexplanation is omitted here. After that, since the IGBTs 101 and 104 arein the ON state, the direction of the current I(20) is reversed and thecurrent I(20) increases. The energy of the power source 10 isaccumulated in the resonance reactor 20.

(Period of Time E3)

When the IGBT 101 is turned off, the current I(20) discharges thesnubber capacitor 122 while charging the snubber capacitor 121. When thevoltage V(102) reaches the zero voltage, the diode 112 is madeconductive. At this time, by turning on the IGBT 102, the zero voltageswitching is performed. Other operations are similar to those for theperiod of time E2 of the step-up operation 2 and their detailedexplanation is omitted here.

The operation for the period of time E3 is symmetrical with that for theperiod of time A3. Subsequently, after the periods of time F3 to H3, theoperation cycle is returned to the period of time A3. Since theoperations for the periods of time F3 to H3 are symmetrical with thosefor the periods of time B3 to D3, their detailed description is omittedhere.

In the step-up operation 3, the durations of the periods of time D3 andH3 are changed. That is, the output electric power is adjusted bychanging the off timing of the IGBTs 101 and 102. If the off timing ofthe IGBTs 101 and 102 is delayed, the circulating currents for theperiods of time A3 and E3 increase and the output electric powerincreases.

As mentioned above, in the step-up operations 1 to 3, the voltageclamping circuit accumulates the energy of the current which is shut offwhen the MOSFETs 201 and 202 are turned off, thereby suppressing thatthe surge voltage is generated in the drain voltage of each of theMOSFETs 201 and 202. Therefore, the voltage clamping circuit has such aneffect that elements of a low withstanding voltage can be used as diodes211 and 212 and MOSFETs 201 and 202.

The voltage has been developed in the resonance capacitor 22 in thedirection adapted to increase the circulating current for the period oftime during which the circulating current flows. Therefore, theresonance capacitor 22 has an effect of increasing the output electricpower.

The foregoing step-down operations 1 and 2 and the foregoing step-upoperations 1, 2, and 3 can be also switched and executed, respectively.The switching of the step-down operations and step-up operations of thebi-directional DC-DC converter 5 will be described hereinbelow.

The switching of the step-down operations 1 and 2 will be described.First, the step-down operation 1 is applied at the small load. Such anoperation that the MOSFET 203 is turned off prior to turning off theIGBT 103, the MOSFET 204 is turned off prior to turning off the IGBT104, and the circulating current is increased is executed. The dutyincreases in association with an increase in load. A time differencebetween the off timing of the IGBT 103 and the off timing of the MOSFET203 and a time difference between the off timing of the IGBT 104 and theoff timing of the MOSFET 204 decrease and those time differences aresoon eliminated. When the load further increases, in order to reduce thecirculating current, such an operation that the MOSFET 203 is turned offafter the IGBT 103 was turned off and the MOSFET 204 is turned off afterthe IGBT 104 was turned off is executed. When the load furtherincreases, the duty increases and soon becomes maximum. In the case offurther increasing the load to a value larger than that in the abovestate, the step-down operation 2 is applied. Specifically speaking, atime which is necessary until the MOSFET 203 is turned off after theIGBT 103 was turned off and a time which is necessary until the MOSFET204 is turned off after the IGBT 104 was turned off are extended.

Subsequently, the switching of the step-up operations 1, 2, and 3 willbe described. First, the step-up operation 1 is applied at the smallload. Such an operation that the IGBT 103 is turned off prior to turningoff the MOSFET 202, the IGBT 104 is turned off prior to turning off theMOSFET 201, and the circulating current is decreased or allowed toreversely flow is executed. The on duties of the MOSFETs 201 and 202increase in association with the increase in load. A time differencebetween the off timing of the MOSFET 202 and the off timing of the IGBT103 and a time difference between the off timing of the MOSFET 201 andthe off timing of the IGBT 104 decrease and those time differences aresoon eliminated. When the load further increases, the on duty reaches anupper limit. The reason why there is an upper limit in the on duty hasalready been described in the explanation of the step-up operation 1. Inthe case of further increasing the load to a value larger than that inthe above state, the step-up operation 2 is applied. Specificallyspeaking, a time which is necessary until the IGBT 103 is turned offafter the MOSFET 202 was turned off and a time which is necessary untilthe IGBT 104 is turned off after the MOSFET 201 was turned off areextended. When the load further increases, the duration of the timereaches an upper limit. The reason why there is an upper limit in theduration of the time has already been described in the explanation ofthe step-up operation 2. In the case of further increasing the load to avalue larger than that in the above state, the step-up operation 3 isapplied.

It is a common point among a plurality of operations is that the IGBTs101 to 104 and the MOSFETs 201 to 204 are operated so as to control thecurrent I(20) flowing in the resonance reactor 20.

As mentioned above, the bi-directional DC-DC converter 5 according tothe embodiment of the invention has such a feature that, in the cases ofboth of the step-down operation and the step-up operation, by switchinga plurality of operations in accordance with a load state, the smallinsulating type bi-directional DC-DC converter in which the highefficiency and high output are obtained even in the case of the smallload can be realized.

The invention is not limited to the foregoing embodiments. Naturally,the invention can be also applied to various circuit constructions inwhich, for example, a current doubler synchronous rectifying circuit maybe used in place of the switching circuit on the low voltage side, theswitching elements of the switching circuit on the low voltage side areconstructed in a full bridge form, and the like.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A bi-directional DC-DC converter which comprises a high voltage sideswitching circuit, connected between a first DC power source and atransformer, for executing an electric power conversion between a directcurrent and an alternating current, a low voltage side switchingcircuit, connected between a second DC power source and saidtransformer, for executing an electric power conversion between a directcurrent and an alternating current, and a control circuit forcontrolling ON/OFF of switching elements included in each of saidswitching circuits and in which an electric power is transmitted andreceived between said first and second DC power sources, wherein saidhigh voltage side switching circuit includes: a first vertical arm inwhich a first upper arm switching element and a first lower armswitching element are serially connected, a second vertical arm in whicha second upper arm switching element and a second lower arm switchingelement are serially connected, a first smoothing capacitor connected inparallel to said first and second vertical arms and said first DC powersource, and an object connected in series which is connected between aseries node of said first upper arm switching element and said firstlower arm switching element and a serial node of said second upper armswitching element and said second lower arm switching element and whichis comprised of a resonance capacitor, a resonance reactor, and aprimary winding of said transformer, said low voltage side switchingcircuit includes: a first switching element group which is connected tosaid second DC power source and a secondary winding of said transformerand includes a plurality of switching elements, a smoothing reactorconnected to said first switching element group and/or the secondarywinding of said transformer, a second smoothing capacitor in which oneend is connected to one end of the switching elements included in saidfirst switching element group and the other end is connected to one endof said smoothing reactor and which is connected in parallel to saidsecond DC power source, and a voltage clamping circuit which isconnected to the switching elements included in said first switchingelement group and has a second switching element group including atleast one switching element and a clamping capacitor, and wherein: saidcontrol circuit is further configured to switch a polarity of DC voltageof the DC power source to be supplied to a series circuit including theresonance capacitor, the resonance reactor and the primary winding ofsaid transformer so that voltage of the clamping capacitor can beapplied to the secondary wiring of said transformer.
 2. A bi-directionalDC-DC converter which comprises a high voltage side switching circuit,connected between a first DC power source and a transformer, forexecuting an electric power conversion between a direct current and analternating current, a low voltage side switching circuit, connectedbetween a second DC power source and said transformer, for executing anelectric power conversion between a direct current and an alternatingcurrent, and a control circuit for controlling ON/OFF of switchingelements included in each of said switching circuits and in which anelectric power is transmitted and received between said first and secondDC power sources, wherein said high voltage side switching circuitincludes: a first vertical arm in which a first upper arm switchingelement and a first lower arm switching element are serially connected,a second vertical arm in which a second upper arm switching element anda second lower arm switching element are serially connected, a firstsmoothing capacitor connected in parallel to said first and secondvertical arms and said first DC power source, and an object connected inseries which is connected between a series node of said first upper armswitching element and said first lower arm switching element and aserial node of said second upper arm switching element and said secondlower arm switching element and which is comprised of a resonancecapacitor, a resonance reactor, and a primary winding of saidtransformer, said low voltage side switching circuit includes: a firstswitching element group which is connected to said second DC powersource and a secondary winding of said transformer and includes aplurality of switching elements, a smoothing reactor connected to saidfirst switching element group and/or the secondary winding of saidtransformer, a second smoothing capacitor in which one end is connectedto one end of the switching elements included in said first switchingelement group and the other end is connected to one end of saidsmoothing reactor and which is connected in parallel to said second DCpower source, and a voltage clamping circuit which is connected to theswitching elements included in said first switching element group andhas a second switching element group including at least one switchingelement and a clamping capacitor, and, wherein: said control circuit isfurther configured to control the first DC power source to dischargeenergy stored in the first DC power source for charging a resonancereactor.